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The density of the slurry mix must be carefully monitored and adjusted to produce the correct outward pressure to prevent the trench walls from collapsing. Slurry walls are typically constructed by starting with a set of concrete guide walls, typically 1 metre (3 ft 3 in) deep and 0.5 metres (1 ft 8 in) thick.
Bentonite layers from an ancient deposit of weathered volcanic ash tuff in Wyoming Gray shale and bentonites (Benton Shale; Colorado Springs, Colorado). Bentonite (/ ˈ b ɛ n t ə n aɪ t / BEN-tə-nyte) [1] [2] is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite.
Without bentonite the perlite separates and floats to the upper part of the slurry. Can be used to achieve a slurry weight as low as 12.0ppg. Bentonite in concentrations of 2–4% is also added to prevent segregation of particles and slurry. Gilsonite – Used to obtain slurry weights as low as 12.0ppg. In high concentrations, mixing is a problem.
A slurry wall is a barrier built under ground using a mix of bentonite and water to prevent the flow of groundwater. A trench that would collapse due to the hydraulic pressure in the surrounding soil does not collapse as the slurry balances the hydraulic pressure.
A lubricant, often bentonite slurry, is injected into this gap. The pressure of the lubricant prevents the gap from collapsing. Depending on the geology a 35mm overcut may create ground subsidence. For road and rail crossings, this 35mm is reduced so prevent more than 10mm subsidence.
The cutter head is filled with pressurised slurry, typically made of bentonite clay that applies hydrostatic pressure to the face. The slurry mixes with the muck before it is pumped to a slurry separation plant, usually outside the tunnel. Slurry separation plants use multi-stage filtration systems that separate spoil from slurry to allow reuse.
Tunnel Construction. Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
The hydromill trench cutter is a specialized type of construction equipment designed to dig the narrow but deep trenches used in the casting of slurry walls. Typically, it is a cutter attachment mounted on a crawler crane base machine, with different types of hose handling systems.